1. Field of the Invention
The present invention involves power control enhancement within mobile communications systems. Moreover, the present invention addresses problems associated with setting power levels. More specifically, the present invention defines a preferred method for setting the power based upon the geographical location of the mobile station.
2. Description of the Prior Art
Power control is key to the successful operation of radio access technology such as Code Division Multiple Access (CDMA). CDMA systems rely on strict control of power at the mobile station to overcome the “near-far problem” that occurs when the interfering signal is significantly stronger than the desired signal and such an interfering signal would then jam the weaker signal. If the signal from a mobile station were to be received at the cell site receiver with too much power, that particular mobile station would then overload that receiver. This would overwhelm the signals from the other mobile stations. The goal of CDMA is to have the signals of all mobile stations arrive at the base station with as exactly similar power as possible. The closer the mobile station is to the cell site receiver, the lower the power necessary for transmission. On the other hand, the further away the mobile station, the greater the power necessary for transmission. Accordingly, two forms of adaptive power control are typically employed in CDMA systems. These include open-loop and closed-loop power control.
Open-loop power control is based on the similarity of loss in the forward and reverse paths. The received power at the mobile station is used as a reference. If it is low, the mobile station is assumed to be far from the base station and transmits with high power. If it is high, the mobile station is assumed to be near the base station and transmits with lower power.
Closed-loop power control is used to adjust the power from the mobile station from the open-loop setting. This adaptive process adjusts the power from the initial open-loop setting to match the power needed for the mobile's current location and the current radio propagation conditions. This is achieved by an active feedback system from the base station to the mobile station. As an example, power control bits may be sent every 1.25 ms to direct the mobile station to increase or decrease its transmitted power by 1 dB. Lack of power control to at least this accuracy greatly reduces the capacity of CDMA systems.
With these adaptive power control techniques, the mobile station transmits only enough power to maintain a link. This results in an average power requirement that is much lower than that for analog systems, which do not usually employ such techniques. CDMA's lower power requirement translates into smaller and lightweight mobile terminals, longer-life of batteries and makes possible, lower-cost handheld computers and wireless computercommunications devices.
Within mobile communications systems and especially such systems having high traffic, the radio power must be kept to the minimum necessary to maintain the desired error rate for the communications link. Using the lowest amount of radio power minimizes the interference to other calls and thereby maximizes the traffic-carrying capability of the radio system.
The total power required for a radio link in the typical terrestrial radio environment is a complicated statistical function of the environment and the distance as is known in the art. Thus the power cannot be directly calculated from the distance, as it is subject to additional losses. These are often summarized as “fading” and “shadowing.” Fading is extra loss due to the combination at the receiver of reflected signals from the environment and the motion of the transmitter or receiver or objects in the environment. Shadowing is extra loss due to obstructions (e.g., buildings) between the transmitter and the receiver. Further details of radio propagation modeling can be found, for example, in Chapter two of Gordon L. Stuber's book “Principles of Mobile Communication” published by Kluwer Academic
While fading loss may vary rapidly with time (due to movement of the mobile or objects in the environment), the shadowing loss is relatively constant with time and only varies (often abruptly) as the mobile moves into or out of the shadowed region. Because the radio channel conditions vary over time due to fading and shadowing, the radio power is typically adjusted with a feedback control loop (as mentioned above) between the transmitter and the receiver. In traditional systems, the transmitter and the receiver exchange signaling messages to adjust the power level to the minimum needed for the desired error ratio. Such requisite levels and ratios are found in the Telecommunications Industry Association (TIA) standards IS-95 or IS-2000. This signaling involves a two-way flow of information between the base station and mobile station in order to send the power control messages. However, this two-way flow is not always possible, particularly during call set-up and for one-way data flows prevalent in data transfer and signaling. In these latter cases, it would be advantageous to have another means of adjusting the power to the optimum level rather than the extensive open and closed loop control processes described above.